A polymer electrolyte membrane fuel cell system has been widely used for vehicles. In the polymer electrolyte membrane, water is used to transfer hydrogen ions (H+) to a membrane disposed between an air electrode and an anode, and a predetermined level of water is maintained within the electrolyte membrane, which is related to performance of a fuel cell.
Under a dry condition in which water is supplied less than the predetermined level, the water content in the electrolyte membrane decreases, which increases resistance of an electrode and deteriorates performance of a fuel cell. Under a flooding condition in which the amount of water is supplied greater than the predetermined level, water vapor may condense in channels through which air and hydrogen gas are supplied to an air electrode and an anode, respectively, and reaction gases may not be supplied to the electrodes, thereby causing substantial drop of a cell voltage of the fuel cell.
In the related art, a conventional method is provided to adjust pressures and supercharging ratios of air and hydrogen gas which are supplied to a fuel cell according to water content in an electrode of a fuel cell or the humidity in the supplied air. Although the method of adjusting the pressure of air has been used effectively, the effect may be insufficient since a range of pressure of air supplied by a compressor or blower may be limited based on surging and chocking conditions and flow rates of air.
Further, in a conventional method of controlling water content in an electrode-membrane, the pressure of air may be elevated and supercharging ratios may be reduced in dry conditions of the fuel cell stack, and the pressure of air may be reduced and the super charging ratios may be elevated in flooding conditions containing high water content in the fuel cell stack.
Such control may be performed based on the following assumptions. Since the saturation vapor pressure of water depends on a temperature, a partial pressure of vapor may be constant at a specific temperature and humidity. Accordingly, when the pressure of supplied air is elevated, the amount of water per unit mass of air to maintain the predetermined level of humidity may decrease. As such, the humidity of supplied air may increase with a decreased amount of water. On the other hand, when the pressure of supplied air is reduced, the amount of water to maintain the predetermined level of humidity may increase.
FIG. 1 shows a performance curve of an exemplary compressor in the related art. a compressor is used to supply air to an air electrode of a fuel cell. The compressor can control the pressure and flow rate of air only within operable ranges due to surging and choking. The operable ranges are directed to an area below the surge curve and above the choking curve in FIG. 1. As illustrated in FIG. 1, according to the related art, the control of the pressure and flow rate of air may be conducted within the operable ranges when the water content in a fuel cell is adjusted the pressure and flow rate of the supplied air. Therefore, the effect may insufficient. Particularly, when the flow rate of air supplied to a fuel stack is reduced, the pressure of air may not be elevated to pressures which are beyond points of the surge curve of FIG. 1 due to the limitation of a system. Furthermore, the pressure of air may be controlled within a limited pressure range under the conditions of the operable range below the surge curve since a minimum operation curve may be determined by a back pressure applied to parts connected to a pipe extended to an air electrode.
The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.